Building an equitorial mount (Some Math)

I want to build a simple equatorial mount, which is to say a mount that can be aimed at the North Celestial Pole, and then turn at the rate of 1 turn per sidereal day. Or 1 turn in 1436.06 minutes. This will match the rate at which the stars move across the sky.

To do this I will need a main gear which will be a wood disk cut on a router of radius R.
This will be driven by a threaded rod with 20 turns per inch turning at 1 rpm. This will move the disk at the rate of 1/2oth of an inch per minute. As such the circumference of the drive wheel must be (1436.06 minutes * 1/20 inches /minute) which is 71.8 inches.

Thus the radius (R) of the drive wheel should be 11.4278 Inches. I expect 11.45 or so will be about as close as I can machine the thing, this should be close enough for visual use and for astrophotoraphy as long as the exposures are short (say 10-15 minutes max).